Bump gunicorn from 21.2.0 to 23.0.0 in /server

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 21.2.0 to 23.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@21.2.0...23.0.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

server/requirements.txt

@@ -115,7 +115,7 @@ flask-script==2.0.6
     # via superdesk-core
 future==1.0.0
     # via python-twitter
-gunicorn==21.2.0
+gunicorn==23.0.0
     # via -r requirements.in
 hachoir==3.0a3
     # via superdesk-core